linux-kernel - Re: [RFC] xfs: fake fallocate success for always CoW inodes

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <aQyz1j7nqXPKTYPT@casper.infradead.org>
Date: Thu, 6 Nov 2025 14:42:30 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Christoph Hellwig <hch@....de>
Cc: Florian Weimer <fweimer@...hat.com>,
	Hans Holmberg <hans.holmberg@....com>, linux-xfs@...r.kernel.org,
	Carlos Maiolino <cem@...nel.org>,
	Dave Chinner <david@...morbit.com>,
	"Darrick J . Wong" <djwong@...nel.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	libc-alpha@...rceware.org
Subject: Re: [RFC] xfs: fake fallocate success for always CoW inodes

On Thu, Nov 06, 2025 at 02:52:12PM +0100, Christoph Hellwig wrote:
> On Thu, Nov 06, 2025 at 02:48:12PM +0100, Florian Weimer wrote:
> > * Hans Holmberg:
> > 
> > > We don't support preallocations for CoW inodes and we currently fail
> > > with -EOPNOTSUPP, but this causes an issue for users of glibc's
> > > posix_fallocate[1]. If fallocate fails, posix_fallocate falls back on
> > > writing actual data into the range to try to allocate blocks that way.
> > > That does not actually gurantee anything for CoW inodes however as we
> > > write out of place.
> > 
> > Why doesn't fallocate trigger the copy instead?  Isn't this what the
> > user is requesting?
> 
> What copy?

I believe Florian is thinking of CoW in the sense of "share while read
only, then you have a mutable block allocation", rather than the
WAFL (or SMR) sense of "we always put writes in a new location".